Dropsheet-duct tape wall in The Martian

[DaveC426913]

TL;DR Summary

How much "weight" is Watney's magical dropsheet actually holding?

While not managing to sleep, I was trying to calculate how deep a pool of water would be to simulate Watney's pressure wall holding in the Hab atmo.

Let's say, generously, the air pressure in the Hab was a mere 0.5 atmos. That's .51kg/cm^2.

Mars is .02 atmos, so effectively vacuum. We can discount it.

So, if we brought it to Earth and turned it on its side and made a 10 foot diamter swimming pool out of it, every square centimetre is holding 500cm^3 of water - or a column 5 metres tall.

So, Watney has built a 10 foot wide suspended pool on Earth - whose bottom is made of magic dropsheet and duct tape, and it is holding up all the water in a pool that's 5 metres deep??

Did I miss a decimal point somewhere?

 

[phinds]

It's magical movie set duct tape.

 

[Ibix]

It's magical movie set duct tape.

Like the Force it is. A light side and a dark side it has. Binds the universe together it does.

But yeah, I agree Dave's calculations. If the guy used pure oxygen (and just lived with the fire hazard) could he reduce pressure to 0.2 bar? That would reduce the swimming pool equivalent to about 2m deep.

 

[kuruman]

What's the matter with you guys? Watney never did the calculation before putting it together and it worked! You missed the point. If you don't know about it, it ceases to be a problem. Same idea as not testing for COVID 19: you don't test, the number of cases goes down. Sheesh!

 

[trainman2001]

What's the matter with you guys? Watney never did the calculation before putting it together and it worked! You missed the point. If you don't know about it, it ceases to be a problem. Same idea as not testing for COVID 19, you don't test, the number of cases goes down. Sheesh!

Yeah! We all know that! Like… it's if you don't take the pregnancy test, you can't be pregnant. Or if you don't have a temperature gauge in your car, it can't overheat. There's lotsa stuff like that.

 

[Filip Larsen]

If you don't know about it, it ceases to be a problem.

Given that nature is in on it as well, I suspect Watneys case is some kind of combination of Somebody else's Problem and Cartoon Physics.

Perhaps the two phenomenon are related via some corollaries, like Cartoon Physics being natures version of SEP. Sort of nature thinks that humans knows what they are doing and stop worrying about getting all the tiny wheel of nature to turn exactly right. I mean, it must be exhausting for nature to be so diligent all the time, right?

On the other hand, given Occam's razor the magic movie duct tape is perhaps the simplest, but less interesting, explanation of this case. Watney clearly have gravity and other aspects of mechanics working OK and nature do punish him in other regards on several occasions so clearly not all is under influence of SEP and CP. And also, magic duct tape is pretty strong, especially the standard grey version.

 

[davenn]

Given that nature is in on it as well, I suspect Watneys case is some kind of combination of Somebody else's Problem and Cartoon Physics.

I like the "Hitchhiker's Guide..." SEP reference, one of my fav's

 

[DrStupid]

But yeah, I agree Dave's calculations. If the guy used pure oxygen (and just lived with the fire hazard) could he reduce pressure to 0.2 bar? That would reduce the swimming pool equivalent to about 2m deep.

That's still too much. According to the Barlow's formula the maximum radius of the green house is given by

##r = \frac{{\sigma \cdot s}}{p}##

with the pressure ##p##, the maximum tensile strength ##\sigma## and the thickness ##s##.

3M 6969 Duct Tape has a thickness of 0.272 mm and a tensile strength at break of 20.7 MPa. With a pressure of 20.265 kPa the maxium radius would be just 27.8 cm. Watney's green house reather looks like 10 meters in diameter. That would require at least 18 layers of duct tape.